Simple graphics language
From ScienceZero
Contents
A simple graphics language for microcontrollers
We start with a very simple reverse polish notation interpreter with a preprocessor stage
Sample code:
to circle 360 for 1 forward 1 right next end
Registers:
0 X (nn.ff) 1 Y 2 Angle 3 Scale 4 Colour 5 Pen up/down 6 Nested If/For counter 7 Local variable pointer
Stack diagrams
( things read from the stack -- things written on the stack ) x - x position y - y position n - number Examples: ( n n -- n ) This means that two numbers were read from the stack, then one number was written back. For example a simple addition reads two numbers and writes back the result. ( a b c -- c b a ) Reading threee values from the stack and write them back in the opposite order.
Definitions
- Function:
- To < function name > < code > End
- Constant:
- Define < constant name > < costant > End
- Macro:
- Macro < macro name > < code > End
Commands
Absolute commands setcolour ( colour -- ) readcolour ( -- colour ) setpos ( x y -- ) readpos ( -- x y ) load ( address -- data ) store ( data address -- ) var ( n -- )( create n variables of value 0 on the return stack ) unvar ( n -- )( delete n variables from the return stack ) ldl ( index -- )( load local variable ) stl ( n index -- )( store local variable ) Relative commands: penup ( -- ) pendown ( -- ) forward ( length -- ) backward ( length -- ) left ( angle -- ) right ( angle -- ) Loops: for ( n -- ) next ( -- ) do ( -- ) again ( -- ) Conditionals: if ( n -- ) else ( -- ) endif ( -- ) Data processing: + ( n n -- n ) ( to + >r 0 r> - - end ) - ( n n -- n ) ( fundamental ) * ( n n -- n ) ( to * 0 do over 1 and if rot dup >r -rot r> + endif rot 1 lsl rot 1 lsr rot over if again endif >r drop -rot drop drop end ) / ( n n -- n ) lsl ( n n -- n ) ( to lsl for dup + next end ) lsr ( n n -- n ) ( to lsr for 31 for dup 0x80000000 and swap 1 lsl swap if 1 + endif next 0x7fffffff and next end ) and ( n n -- n ) ( fundamental ) or ( n n -- n ) ( to or over not and + end ) eor ( n n -- n ) ( to eor over over or -rot and not and end ) not ( n -- n ) ( to not >r -1 r> - end ) Stack manipulation: drop ( n -- ) ( to drop if endif end ) dup ( n -- n n ) ( fundamental ) swap ( a b -- b a ) ( to swap dup >r >r dup r> eor dup >r eor r> r> eor end ) over ( a b -- a b a ) ( to over >r dup r> swap end ) rot ( a b c -- b c a ) ( to rot >r swap r> swap end ) -rot ( a b c -- c a b ) ( to -rot swap >r swap r> end ) >r ( n -- ) ( fundamental ) r> ( -- n ) ( fundamental )
Optimal stack reordering in Forth
Compiler
Stage 1
- Definitions are expanded
Stage 2
- Macros are executed
Stage 3
- Functions are added to the dictionary
Stage 4
- All words are looked up in the dictionary and the opcode stored in the program array
- The address of each function is stored in the function jump table
The dictionary
The dictionary used to convert a command into an opcode. It is a zero terminated string like "setcolour readcolour setpos readpos ",0 that is searched linearly from start to end. A space marks the end of a word. The first compiler pass adds all functions to the end of this table.
The function jump table
This table contains the address of each function, in the same order as the functions appear in the dictionary. It is filled in during the second pass when the code position is known.
Interpreter
The opcode pointer always ppoints to the next byte to be processed.
do read opcode switch opcode case 0 push 0 case 1-4 read byte(s) value = byte0 or (byte1<<8) or (byte2<<16) or (byte3<<24) push value case 5 PC = (opcodepointer + 1) and not 1 (execute inline thumb2 ARM code at even address) case command execute command case function pushr opcodepointer call jumptable(opcode) end switch again
Opcodes: 0 load 0 1 load next byte on to the stack 2 load the next two bytes 3 load the next three bytes 4 load the next four bytes 5 execute in line THUMB2 code 6-15 reserved 16-255 commands